1. Field of the Invention
The present invention relates to a crucible, and a growth method of polycrystal silicon using the crucible.
2. Description of the Related Art
Polycrystal silicon, which is used, for example, in production of square wafers for solar cells, is grown by a cast method in which a solid material silicon is melted and then coagulated. The growth of polycrystal silicon is typically performed using a crucible shown in FIG. 6.
A crucible 30 is made of silica (SiO2), for example. The crucible 30 has predetermined inner dimensions and thickness and is in the shape of a hollow rectangular parallelepiped having an opening at an upper portion thereof. The crucible 30 is produced by coating an inner surface thereof with silicon nitride followed by baking.
The thus-constructed crucible 30 is used to grow polycrystal silicon. As shown in FIG. 6, a solid material silicon 20 is loaded into the crucible 30. When it is assumed that the crucible 30 has an inner dimension of 70 cm, a thickness of 1.5 cm, and a height of 48 cm, about 250 kg of the solid material silicon 20 can be loaded into the crucible 30, for example.
The crucible 30 in which the material silicon 20 is loaded is heated in a furnace at about 1420xc2x0 C., which is the melting point of silicon, for about 4 hours. The material silicon 20 loaded into the crucible 30 is melted. In this case, the material silicon 20 loaded into the crucible 30 is all melted as shown in FIG. 7, so that the height of the material silicon 20 becomes about one half of the height of the crucible 30. Thereafter, the melted material silicon 20 in the crucible 30 is cooled for about 15 hours to be coagulated, thereby obtaining a polycrystal silicon ingot 21.
As described above, the crucible 30 is typically in the shape of a rectangular parallelepiped having a predetermined inner dimension. Even though the crucible 30 is filled with the solid material silicon 20, since there are a number of interstices within the solid material silicon 20, it is not possible to efficiently load the material silicon 20 into the crucible 30. Therefore, when the material silicon 20 is melted, the volume of the crucible 30 is reduced so that the height of the material silicon 20 becomes only about one half of the height of the crucible 30. The coagulated polycrystal silicon ingot is formed only to about one half of the height of the crucible 30.
For example, as described above, when the crucible 30 has an inner dimension of 70 cm, a thickness of 1.5 cm, and a height of 48 cm, a polycrystal silicon ingot having only a height of 24 cm in the shape of a rectangular parallelepiped is obtained. Therefore, in order to provide a polycrystal silicon ingot having a predetermined height, a crucible having a depth twice as great as the height of the ingot is required.
The melted material silicon 20 expands when it is coagulated. Therefore, in this case, a great stress acts on the crucible 30. The crucible 30 is typically made of baked silica. When the crucible 30 is subjected to a high temperature of 1420xc2x0 C. which is the melting point of silicon, since silica is crystallized, the strength of the crucible 30 is reduced so that cracks are likely to occur in the crucible 30. As a result, cracks occur in the crucible 30 every time the material silicon 20 is melt and then coagulated, whereby the crucible 30 may eventually become unusable, causing an economical problem.
According to an aspect of the present invention, a crucible used in the growth of polycrystal silicon by a cast method comprises a crucible body for, when solid material silicon is melted, containing the melted material silicon, and a material holder provided on the crucible body, for holding further material silicon on the material silicon loaded into the crucible body.
In one embodiment of this invention, the material holder is detachable from the crucible body.
In one embodiment of this invention, an inner space of the material holder for holding the material silicon is gradually increased toward an upper portion of the material holder.
In one embodiment of this invention, a lower portion of the material holder is inserted into and held by an upper portion of the crucible body.
In one embodiment of this invention, a size of the crucible body is designed so that when the material silicon loaded into the crucible body and the material holder is melted, an upper surface of the melted material silicon is positioned close to the upper portion of the crucible body.
In one embodiment of this invention, the material holder is made of a material different from that of the crucible body.
In one embodiment of this invention, the crucible body is made of baked silica, and the material holder is made of a material containing carbon.
In one embodiment of this invention, a groove is provided at a lower portion of the material holder, the groove fits an upper portion of the crucible body, the material holder is held by fitting the groove to the crucible body.
According to another aspect of the present invention, a method for growing polycrystal silicon, compries the steps of loading a solid material silicon into the crucible body and the material holder of the above-described crucible, heating the loaded material silicon so as to be melted, and coagulating the melted material silicon in the crucible body.
Thus, the invention described herein makes possible the advantages of providing: a crucible with which polycrystal silicon can be efficiently grown and which can be used for a long time and is therefore economical; and a method for efficiently growing polycrystal silicon using the crucible.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.